Computer equipment and energy efficient lighting are becoming increasingly important types of electrical loads for many different facilities including schools, hospitals, water treatment plants, prisons, commercial establishments and industrial plants. As a result, computer equipment and energy efficient lighting are also becoming an increasingly greater burden on utility power systems. Moreover, many of these facilities are required to provide emergency backup systems to maintain operation of important computer equipment and energy efficient lighting loads, as well as various other electrical loads, during utility power failures such as outages (blackouts) or low voltage situations (brownouts).
Many computer equipment and energy efficient lighting loads, such as electronic compact fluorescent lamps, interface with the utility power through rudimentary power inverters consisting of full bridge rectifiers and DC bus capacitors. These economical circuits draw current at the peak of the utility AC voltage and produce current harmonics at utility frequencies. Other types of energy efficient lighting, such as compact fluorescent lamps, use series reactors to interface with utility power. Series reactors, however, also produce current harmonics due to the non-linear characteristics of the fluorescent lamps.
To overcome the problem of drawing current without introducing additional harmonics, solid-state ballasts and some computer equipment use higher performance inverters to interface with the utility power. Ideally, these power inverters maintain constant voltage, current, and power to the load regardless of input voltage. To satisfy these conditions, however, input power (less losses) must be held constant thereby requiring input current to increase upon decreasing input voltage. As a result, such inverters have input characteristics equivalent to a dynamic negative resistor.
Various systems and methods are known to provide backup power in the event of utility power failures. Among the most common are internal combustion engine driven generators. Such generators, however, can suffer from space requirement, noise, ventilation and maintenance problems.
To overcome many of the problems associated with backup generators, solid state sinewave inverters are commonly utilized to provide alternate power in the event of utility power failures. This is especially true where computer equipment and energy efficient lighting type loads are commonly found. Solid state inverters operate on the principle of electronically inverting a DC input voltage to produce an AC output voltage. There are many ways of performing this function each of them having unique characteristics.
Square wave inverters use a transformer and two or four semiconductor switches to produce a square wave voltage at the switching frequency. The output voltage is proportional to the input voltage and the turn ratio of the transformer. These inverters are characteristically very low cost and only work with loads not sensitive to voltage waveform.
Ferroresonant inverters consist of a square wave inverter and a ferroresonant transformer. Ferroresonant transformers have a current limiting reactor and an AC capacitor that is tuned to the driving frequency of the inverter. Such a series resonant circuit is designed to saturate the magnetic core, thereby regulating the output voltage. The ferroresonant transformer extracts the fundamental component from the square wave to produce a regulated sinewave. However, because the ferroresonant transformer must filter the fundamental component, its output impedance is relatively high compared to that of alternative systems and methods. Such a high output impedance has an adverse effect and can cause some instability in constant power load devices.
Pulse-Width Modulated (PWM) inverters consist of a modified square wave inverter and a low pass output filter. The power circuit of such an inverter is similar to that described above, but the switching rate is increased to lower the harmonic concentration near the fundamental component. The strategy is to suppress lower order harmonics with pulse width modulation, and to suppress higher order harmonics with the low pass filter. The switching method and rate determine the harmonic free band near the fundamental component, the size of the filter, and system losses.
PWM inverters, however, still suffer from various problems associated with powering non-linear loads, such as computers, solid-state ballasts and compact fluorescent lamps, which either generate current harmonics at the driving frequency, or produce loading characteristics of negative resistance. As a result, there is a need for a system and method for controlling a PWM modulated inverter for use with such non-linear loads and in particular lighting loads, as well as with linear loads generally.
Such a system and method may employ a PWM inverter having a switching pattern optimized for lower order harmonic elimination, an output filter whose response has been tailored to the PWM waveform, and a control algorithm which integrates a digital control inside an analog closed-loop control. Such a system and method for controlling a DC to AC voltage inverter would allow unrestricted operation of both linear and non-linear electrical loads, such as computer equipment and energy efficient lighting, in the event of utility power failure.